hardMultiple ChoiceObjective-mapped
300-410 Practice Question: Redistributes OSPF routes into BGP on router R1
A network engineer redistributes OSPF routes into BGP on router R1. R1 has: router bgp 65001, redistribute ospf 1 route-map RM_OSPF. The route-map RM_OSPF sets metric 100. OSPF routes from area 0 are redistributed, but routes from area 1 are missing in BGP. R1 shows: show ip ospf database shows area 1 routes as inter-area (IA). What is the root cause?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The route-map RM_OSPF uses match ip address prefix-list PL_INTRA that only permits prefixes from area 0, filtering inter-area routes from area 1.
By default, when redistributing OSPF into BGP, only intra-area and inter-area routes are redistributed, but external routes (type 5/7) are not. However, the missing routes are inter-area (IA) from area 1, which should be redistributed. The issue is that the route-map RM_OSPF may have a match clause that inadvertently filters IA routes, or the redistribute command may be missing the match internal keyword. In this case, the redistribute ospf 1 command without any match option defaults to match internal, which includes intra-area and inter-area. But if the route-map has a match ip address prefix-list that only permits specific prefixes, it could filter others. The root cause is that the route-map RM_OSPF uses match ip address prefix-list PL_INTRA which only permits prefixes from area 0, thus filtering inter-area routes from area 1.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The route-map RM_OSPF uses match ip address prefix-list PL_INTRA that only permits prefixes from area 0, filtering inter-area routes from area 1.
Why this is correct
Inter-area routes are internal but the prefix-list restricts to area 0 prefixes only.
- ✗
OSPF inter-area routes are not redistributed into BGP by default; you must use redistribute ospf 1 match internal external.
Why it's wrong here
Default redistribution includes internal routes (intra-area and inter-area).
- ✗
The metric 100 set in the route-map is too low, causing BGP to ignore the routes.
Why it's wrong here
Metric does not cause routes to be ignored; it only affects path selection.
- ✗
The OSPF process ID is incorrect; redistribution uses process ID 1 but area 1 routes are from a different process.
Why it's wrong here
All OSPF routes from process 1 are considered, regardless of area.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
BGP Prefix Filtering
BGP Prefix Filtering is the practice of controlling which network routes (prefixes) a router accepts or advertises to its BGP neighbors, preventing unwanted or harmful routes from spreading across the internet.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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